/Engine/Plugins/Media/ElectraUtil/Source/ElectraHTTPStream/Public/ElectraHTTPStream.h
Adds a request header. Only one header of a type is allowed. If one already exists it will be replaced with the new value unless bAppendIfExists is true which will append the value to the existing value with a ", " sequence. An existing header can be removed by passing an empty value with bAppendIfExists set to false. This may not be useful since you can always just not add the header to begin with.
void AddHeader ( const [FString](API\Runtime\Core\Containers\FString) & Header, const [FString](API\Runtime\Core\Containers\FString) & Value, bool bAppendIfExists )